The present invention relates to a heterojunction bipolar transistor, and in particular, to a bipolar transistor that has a collector/base heterojunction.
As a conventional heterojunction bipolar transistor, there has conventionally been known a double heterojunction bipolar transistor (DHBT) such that a graded layer whose bandgap linearly changes in the direction of thickness thereof is interposed between a collector and a base in order to remove band discontinuity at a heterojunction located between the collector and the base (the junction being referred to as a collector/base heterojunction hereinafter) (Q. M. Zhang et al., xe2x80x9cAnalysis of the Emitter-Down Configuration of Double-Heterojunction Bipolar Transistorxe2x80x9d, IEEE Transactions On The Electron Devices, vol. 39 No. 10, pp. 2220-2229, 1992). As another double heterojunction bipolar transistor, there is known one such that a setback layer that has a high concentration and a narrow bandgap is interposed between the collector and the base in order to lower a barrier at the collector/base heterojunction (W-C. Liu et al., xe2x80x9cApplication of xcex4-doped wide-gap collector structure for high-breakdown and low-offset voltage transistorsxe2x80x9d, Applied Physics Letters, vol. 73 No. 10, pp. 1397-1399, 1998).
The graded layer and setback layer reduce to some extent the barrier viewed from mobile charge carriers. However, these layers cannot completely remove the barrier. This becomes significant particularly when a bias voltage is low, i.e., when the transistor is operated under bias conditions near a saturation region.
For example, FIG. 3A shows a calculated energy band diagram of an npn type DHBT provided with the aforementioned graded layer when a bias set sufficiently away from the saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=2 V) is applied to the DHBT. FIG. 3B shows a calculated energy band diagram of the same DHBT operated with a bias set near the saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=0.1 V). This npn type DHBT is constructed of an n-type Al0.3Ga0.7As emitter layer (having a thickness of 800 xc3x85 and an impurity concentration n=5xc3x971017 cmxe2x88x923) 31, an AlxGa1xe2x88x92xAs emitter graded layer (having a thickness of 200 xc3x85, an impurity concentration n=5xc3x971017 cmxe2x88x923 and an Al mole fraction x=0.3xe2x86x920) 32, a p+-type GaAs base layer (having a thickness of 800 xc3x85 and an impurity concentration n=4xc3x971019 cmxe2x88x923) 33, an n-type AlxGa1xe2x88x92xAs graded layer (having a thickness of 500 xc3x85, an impurity concentration n=2xc3x971016 cmxe2x88x923 and an Al mole fraction x=0xe2x86x920.2) 34, an n-type Al0.2Ga0.8As collector layer (having a thickness of 6000 xc3x85 and an impurity concentration n=2xc3x971016 cmxe2x88x923) 35, an AlxGa0.8As graded layer (having a thickness of 500 xc3x85, an impurity concentration n=2xc3x971016 cmxe2x88x923 and an Al mole fraction x=0.2xe2x86x920) 36 and an n-type GaAs subcollector layer (having a thickness of 1000 xc3x85 and an impurity concentration n=5xc3x971018 cmxe2x88x923) 37. The arrow xe2x80x9cxe2x86x92xe2x80x9d between the numbers of the Al mole fraction x means that the value of the Al mole fraction x linearly changes in the direction of thickness of the layer. FIGS. 3A and 3B show that a collector/base heterojunction barrier 39 cannot be sufficiently reduced by the graded layer when the bias voltage is low.
FIG. 4A shows a calculated energy band diagram of an npn type DHBT provided with the aforementioned setback layer when a bias set sufficiently away from a saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=2 V) is applied to the DHBT. FIG. 4B shows a calculated energy band diagram of the same DHBT operated with a bias set near the saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=0.1 V). This npn type DHBT is constructed of an n-type Al0.3Ga0.7As emitter layer (having a thickness of 800 xc3x85 and n=5xc3x971017 cmxe2x88x923) 41, an AlxGa1xe2x88x92xAs emitter graded layer (having a thickness of 200 xc3x85, an impurity concentration n=5xc3x971017 cmxe2x88x923 and an Al mole fraction x=0.3xe2x86x920) 42, a p+-type GaAs base layer (having a thickness of 800 xc3x85 and an impurity concentration n=4xc3x971019 cmxe2x88x923) 43, an n-type GaAs setback layer (having a thickness of 500 xc3x85, an impurity concentration n=2xc3x971017 cmxe2x88x923) 44, an n-type Al0.2Ga0.8As collector layer (having a thickness of 6000 xc3x85 and an impurity concentration n=2xc3x971016 cmxe2x88x923) 45, an n-type Al0.2Ga0.8As collector layer (having a thickness of 500 xc3x85 and an impurity concentration n=2xc3x971018 cmxe2x88x923) 46 and an n-type GaAs subcollector layer (having a thickness of 1000 xc3x85 and an impurity concentration n=5xc3x971018 cmxe2x88x923) 47. FIGS. 4A and 4B show that a collector/base heterojunction barrier 49 cannot be sufficiently reduced by the setback layer when the bias voltage is low.
FIG. 5A shows a calculated energy band diagram of an npn type DHBT provided with a xcex4-doped layer in addition to the aforementioned setback layer when a bias set sufficiently away from a saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=2 V) is applied to the DHBT. FIG. 5B shows a calculated energy band diagram of the same DHBT operated with a bias set near the saturation region (a base-emitter voltage Vbe=1.4 V and a collector-emitter voltage Vce=0.1 V). This npn type DHBT is constructed of an n-type Al0.3Ga0.7As emitter layer (having a thickness of 800 xc3x85 and n=5xc3x971017 cmxe2x88x923) 51, an AlxGa1xe2x88x92xAs emitter graded layer (having a thickness of 200 xc3x85, n=5xc3x971017 cmxe2x88x923 and an Al mole fraction x=0.3xe2x86x920) 52, a p+-type GaAs base layer (having a thickness of 800 xc3x85 and an impurity concentration n=4xc3x971019 cmxe2x88x923) 53, an n-type GaAs setback layer (having a thickness of 500 xc3x85, an impurity concentration n=2xc3x971016 cmxe2x88x923) 54, a xcex4-doped layer (donor 2-dimensional doping density Ns=5xc3x971011 cmxe2x88x922) 55, an n-type Al0.2Ga0.8As collector layer (having a thickness of 6000 xc3x85 and an impurity concentration n=2xc3x971016 cmxe2x88x923) 56, an n-type Al0.2Ga0.8As collector layer (having a thickness of 500 xc3x85 and an impurity concentration n=2xc3x971018 cmxe2x88x923) 57 and an n-type GaAs subcollector layer (having a thickness of 1000 xc3x85 and an impurity concentration n=5xc3x971018 cmxe2x88x923) 58. FIGS. 5A and 5B show that a collector/base heterojunction barrier 59 cannot be sufficiently reduced when the bias voltage is low even if the xcex4-doped layer is provided in addition to the setback layer.
Accordingly, an object of the present invention is to provide a heterojunction bipolar transistor including a collector/base heterojunction such that mobile charge carriers can pass through a collector region from a base region without encountering any barrier, therefore achieving a high operating efficiency.
In order to achieve the above object, the present invention provides a heterojunction bipolar transistor having an emitter region, a base region, a collector region and a subcollector region, which are sequentially arranged in one direction, wherein the collector region includes a plurality of adjacent sub-regions with respect to a thickness direction in which mobile charge carriers move, an energy bandgap in each of the sub-regions is either constant or linearly changes with a position in the thickness direction, an energy band edge where the mobile charge carriers in the collector region run is continuous between the adjacent sub-regions, and a 2-dimensional or quasi-2-dimensional charge layer is formed at an interface between the adjacent sub-regions so as to compensate a quasi-electric field caused by differences in electron affinity and energy bandgap between the adjacent sub-regions.
In the above invention, the term xe2x80x9c2-dimensional charge layerxe2x80x9d means a charge layer that has a thickness of zero, while the term xe2x80x9cquasi-2-dimensional charge layerxe2x80x9d means a charge layer that has a finite thickness.
The collector region of the heterojunction bipolar transistor of the present invention is divided into a plurality of sub-regions with respect to the thickness direction in which the mobile charge carriers move. The energy bandgap of each of the sub-regions is constant or linearly changes with the position in the thickness direction, and the energy band edge where the mobile charge carriers in the collector region run is continuous between the sub-regions. That is to say, electrons in the conduction band are the mobile charge carriers in the case of a npn type bipolar transistor, and the conduction band edge where the electrons run is continuous. Holes in the valence band are the mobile charge carriers in the case of a pnp type bipolar transistor, and the conduction band edge where the holes run is continuous. With this arrangement, a continuous potential profile is obtained due to the mobile charge carriers that cross the collector region in the thickness direction. Even if a graded layer in which its energy bandgap linearly changes is inserted between the sub-regions that have mutually different energy bandgaps with the position in the thickness direction, then the insertion of the graded layer is solely insufficient for giving a smooth and barrier-free potential to the mobile charge carriers. This is because a quasi-electric field is generated depending on differences in electron affinity and energy bandgap between the sub-regions. According to the present invention, the 2-dimensional or quasi-2-dimensional charge layer is formed at the interface between the adjacent sub-regions so as to compensate the aforementioned quasi-electric field. By inserting the 2-dimensional charge density between the sub-regions, the energy band edge is flattened. This arrangement reduces a barrier height of the mobile charge carriers and improves an operating efficiency. In this stage, it is preferable to sufficiently flatten the energy band edge by appropriately inserting the 2-dimensional charge density between the sub-regions so as to completely compensate the quasi-electric field, substantially removing the barrier of the mobile charge carriers. In particular, by making the entire collector region have a structure of the sub-regions according to the present invention, the mobile charge carriers is allowed to pass through the collector region from the base region without substantially encountering any barrier. In the above case, a specifically high operating efficiency can be achieved.
In one embodiment of the present invention, the heterojunction bipolar transistor is an npn type transistor, the energy band edge of a conduction band is continuous between the adjacent sub-regions, and a 2-dimensional charge density Ns of the 2-dimensional or quasi-2-dimensional charge layer is set so as to satisfy a relationship shown by a following expression (1):
xe2x80x83Ns=(xcex51d"khgr"1/dzxe2x88x92xcex52d"khgr"2/dz)/qxe2x80x83xe2x80x83(1)
where q is an electron charge, "khgr"1 and "khgr"2 are an electron affinity of a first sub-region and a second sub-region between which the charge layer is interposed, xcex51 and xcex52 are a permittivity at an interface between the first sub-region and the second sub-region, and z is a coordinate system which is perpendicular to the interface between the first sub-region and the second sub-region and in which a direction directed from the first sub-region to the second sub-region is positive.
The fact that Ns is positive means that the electric charge layer is constructed of acceptor ions, and the fact that Ns is negative means that the electric charge layer is constructed of donor ions.
In general, the condition required for unimpeded movement of the mobile charge carriers through the semiconductor having specified spatial charges is that differentiation of the quasi-electric field F by the position z i.e. differential dF/dz is continuous throughout the entire region. With regard to the conduction band of the semiconductor, the quasi-electric field Fc of the conduction band is expressed by:
Fc=xe2x88x92d"khgr"/dzxe2x80x83xe2x80x83(5)
where "khgr" is the electron affinity. Therefore, it can be understood that discontinuities of dFc/dz at the boundaries of the sub-regions are canceled by the insertion of the 2-dimensional or quasi-2-dimensional charge layer, which has the 2-dimensional charge density Ns expressed by the aforementioned equation (1), between the sub-regions. Therefore, according to this embodiment, the mobile charge carriers can pass through the collector region from the base region without encountering the barrier in the npn type heterojunction bipolar transistor, and this can consequently achieve a high operating efficiency.
In one embodiment of the present invention, the plurality of sub-regions of the npn type heterojunction bipolar transistor are made of AlxGa1xe2x88x92xAs, an Al mole fraction x of each of the sub-regions is constant or linearly changes with the position in the thickness is direction, the Al mole fractions x of the adjacent sub-regions are coincident with each other at the interface between the adjacent sub-regions, a 2-dimensional or quasi-2-dimensional charge layer is formed the interface of the adjacent sub-regions, and the 2-dimensional charge density Ns of the 2-dimensional or quasi-2-dimensional charge layer is set so as to satisfy a relationship shown by a following expression (2):
Ns=0.8xcex5(dx1/dzxe2x88x92dx2/dz)/qxe2x80x83xe2x80x83(2)
where q is the electron charge, x1 and x2 are an Al mole fraction of the first sub-region and the second sub-region between which the charge layer is interposed, xcex5 is a permittivity at the interface between the first sub-region and the second sub-region, and z is the coordinate system which is perpendicular to the interface between the first sub-region and the second sub-region and in which a direction directed from the first sub-region to the second sub-region is positive.
If the sub-region is constructed of AlxGa1xe2x88x92xAs (GaAs if x=0), then the aforementioned equation (1) can be rewritten into the equation (2) by the Al mole fraction x of AlxGa1xe2x88x92xAs. This is because the electron affinity "khgr"(x) of AlxGa1xe2x88x92xAs is experimentally expressed by the Al mole fraction x as follows:
"khgr"(x)="khgr"0xe2x88x920.8xxe2x80x83xe2x80x83(6)
where "khgr"0 is the electron affinity of GaAs when x=0. Therefore, according to this embodiment, the mobile charge carriers can pass through the collector region from the base region without encountering the barrier in the npn type heterojunction bipolar transistor whose collector region includes a GaAs/AlGaAs-based material, and this can consequently achieve a high operating efficiency.
In one embodiment of the present invention, the heterojunction bipolar transistor is a pnp type transistor, the energy band edge of a valence band is continuous between the adjacent sub-regions, and a 2-dimensional charge density Ns of the 2-dimensional or quasi-2-dimensional charge layer is set so as to satisfy a relationship shown by a following expression (3):
Ns=(xcex51d"khgr"1/dzxe2x88x92xcex52d"khgr"2/dz+(dEG1/dzxe2x88x92dEG2/dz)/q)/qxe2x80x83xe2x80x83(3)
where q is an electron charge, "khgr"1 and "khgr"2 are an electron affinity of a first sub-region and a second sub-region between which the charge layer is interposed, xcex51 and xcex52 are a permittivity at an interface between the first sub-region and the second sub-region, z is a coordinate system which is perpendicular to the interface of the first sub-region and the second sub-region and in which a direction directed from the first sub-region to the second sub-region is positive, and EG1 and EG2 are an energy gap of the first sub-region and second sub-region.
The fact that Ns is positive means that the electric charge layer is constructed of acceptor ions, and the fact that Ns is negative means that the electric charge layer is constructed of donor ions.
As already described hereinabove, the condition required for the unimpeded movement of mobile charge carriers through the semiconductor having specified spatial charges is that differentiation of the quasi-electric field F by the position z i.e. differential dF/dz is continuous throughout the region. With regard to the valence band of the semiconductor, the quasi-electric field Fv of the valence band is expressed by:
Fv=xe2x88x92d("khgr"+EG/q)/dzxe2x80x83xe2x80x83(7)
where "khgr" is the electron affinity and EG is the energy bandgap. Therefore, it can be understood that the discontinuities of dFv/dz at the boundaries of the sub-regions are canceled by the insertion of the 2-dimensional or quasi-2-dimensional charge layer which has the 2-dimensional charge density Ns expressed by the aforementioned equation (3), into the interface between the sub-regions. Therefore, according to this embodiment, the mobile charge carriers can pass through the collector region from the base region without encountering any barrier in the pnp type heterojunction bipolar transistor, and this can consequently achieve a high operating efficiency.
In one embodiment of the present invention, the plurality of sub-regions of the pnp type heterojunction bipolar transistor are made of AlxGa1xe2x88x92xAs, an Al mole fraction x of the sub-region is constant or linearly changes with the position in the thickness direction, the Al mole fractions x of the adjacent sub-regions are coincident with each other at the interface between the adjacent sub-regions, a 2-dimensional or quasi-2-dimensional charge layer is formed on the interface between the sub-regions, and the 2-dimensional charge density Ns of the charge layer is set so as to satisfy a relationship shown by a following expression (4):
Ns=0.45xcex5(dx1/dzxe2x88x92dx2/dz)/qxe2x80x83xe2x80x83(4)
where q is the electron charge, x1 and x2 are an Al mole fraction of the first sub-region and the second sub-region between which the charge layer is interposed, xcex5 is an permittivity at the interface between the first sub-region and the second sub-region, and z is a coordinate system which is perpendicular to the interface between the first sub-region and the second sub-region and in which a direction directed from the first sub-region to the second sub-region is positive.
If the sub-region is constructed of AlxGa1xe2x88x92xAs (GaAs if x=0), then the aforementioned equation (3) can be rewritten into the equation (4) by the Al mole fraction x of AlxGa1xe2x88x92xAs. This is because the electron affinity "khgr"(x) of AlxGa1xe2x88x92xAs and the band gap E(x) are experimentally expressed by the Al mole fraction x as follows:
"khgr"(x)="khgr"0xe2x88x920.8xxe2x80x83xe2x80x83(6)
and
E(x)=EG0+1.247xxe2x80x83xe2x80x83(8)
where "khgr"0 is the electron affinity of GaAs when x=0 and EG0 is the band gap of GaAs when x=0. Therefore, according to this embodiment, the mobile charge carriers can pass through the collector region from the base region without encountering the barrier in the pnp type heterojunction bipolar transistor whose collector region includes a GaAs/AlGaAs-based material, and this can consequently achieve a high operating efficiency.
In one embodiment of the present invention, the collector region of the npn type heterojunction bipolar transistor is comprised of the first sub-region located on a base layer side and the second sub-region located on a subcollector side, and the Al mole fraction x of each of the sub-regions is set so as to linearly increase toward the second sub-region within the first sub-region and linearly decrease toward the subcollector region within the second sub-region.
According to the embodiment of the present invention, the barrier at the interface between the first and second sub-regions made of AlxGa1xe2x88x92xAs can be removed from the npn heterojunction bipolar transistor whose collector region is constructed of the GaAs/AlGaAs-based material. Therefore, the mobile charge carriers can pass through the collector region from the base region without encountering the barrier, and this can consequently achieve a high operating efficiency. The collector structure which has a high operating efficiency and in which the mobile charge carriers can pass through the collector region from the base region without encountering any barrier can be provided with a relatively simplified structure, and this provides great advantages in terms of fabrication.